Just try and enjoy the experience and sense of achievement in taking part and remember that the pride will last longer than the run.
There are bound to be a lot of people of all shapes and sizes going at different speeds, and some won’t make it to the finish line.
You can gain something from the atmosphere of the event, with other participants and supporters to motivate you. And don’t forget the marshals, many of whom are probably associated with a running club - give them a smile and a thank you, they are sure to give you some encouragement.
There are different plans for ultramarathon training, and it probably depends on the distance you're aiming for. Like most marathon training plans you wouldn't do the distance in one training run, but might cover it over a couple of days.
A 50km is little more than a marathon, but 100km is quite different and you can expect to run slowly and get used to eating and drinking. Personally I've trained on consecutive days, and for time rather than distance, i.e. 4 hour long run on Saturday then 3 hours on Sunday, generally at about 6min/km or 10km/h. The other consideration is time on your feet, so include some long walks too. A good strategy for the event is to walk up hills, run down them, and jog on the level.
If you give yourself enough time to train, the answer is yes.
If you’re thinking about a 50km ultra, then you’re almost doing that distance in a week, and half of it on your long run, so it should be fairly easy for you.
Training plans are quite varied for ultramarathons, but some recommend time on your feet (walking and running time), and running on consecutive days. Three runs a week is probably not unreasonable as you will need rest and recovery which could be cross-training, strength or flexibility sessions.
I’ve used SIS gels as you don’t need water, and High 5 gels which are smaller to carry but should be taken with water. Both seem fine.
I usually drink beetroot juice for a few days before races, and take a beet-it shot or cereal bar before a race. I don’t know how much difference it makes to performance, if any. It’s inexpensive and one of your 5-a-day so it can’t be bad.